This invention relates to a method of measuring web tension in paper or foils and means for carrying out the method and especially to such a method and means responsive to transverse oscillations in the web.
In the production or treatment of different elongated members, e.g. printing of paper, plastic or other types of foils, long webs of the member are usually fed between guide rollers, which direct and support the web. It is of the greatest importance that the web tension of the members be kept under control, and preferably as constant as possible, because a change in web tension in one part of the web will give rise to reactions along the whole web. It is also of the utmost importance that the web tension is maintained at the same level across the whole web, because the web can otherwise be warped, with the risk of quilling, and other unfavorable effects. Therefore, it is desirable to continuously detect the web tension at different places along a paper web or the like, and preferably at several different places across the web at the same time. When continuously detecting the web tension, it is possible to directly apply a control when a change in the web tension is dictated.
The most common type of web tension meter in modern web machines having guide rollers, especially printing machines, includes some type of load cells, on which both ends of the rollers are mounted. When the web is deflected by the guide rollers, the forces on the load cells can be utilized for determination of the web tension. In order that the cells might measure rapid variations of the web tension, the weight of the rollers should be as low as possible. Mechanical vibrations will always arise, however, when the rollers rotate, which disturb the signal measured, and therefore, the rollers generally are made rigid to reduce these vibrations, which in turn results in heavy rollers, especially for broad or wide machines. Consequently, the part of the force on the load cells resulting from the web tension is very small relative to the weight of the rollers. Therefore, the required sensitivity of the load cell is very great, which causes great problems.
Great efforts have been made to find a solution of the above-mentioned problems. There are several known devices in which the whole paper web between two rollers is caused to include transverse vibrations. The resonant frequency of a paper web has a very special and quite definite relationship to the tension of the web. This relationship can be written: ##EQU1## WHERE F = THE RESONANT FREQUENCY
T = the tension of the web PA1 n = the overtone for which the web is in resonance PA1 .tau. = the length between the points of support of the web, and PA1 m = mass per surface unit of the web material.
If n, .tau., and m are constants, the relationship is simplified to EQU f = K.sqroot.T
in this connection, the paper web can essentially be compared to a string on a musical instrument. The resonant frequency of the string, as is well-known, increases in accordance with the amount that the string is stretched. The resonant frequency of the paper web is consequently indicated in accordance with these known devices and the resonant frequency is a measure of the web tension.
There are also prior art devices, where an intermittent oscillation is applied to a part of the web, and the travel time required for the oscillation to reach another point on the web is indicated. This travel time has a special relationship to the web tension.
Both of the above methods are sensitive to external disturbances, such as noise. Because they are to be used in a very noisy environment, the measuring device must be shielded from the noise at the measuring place, which is both troublesome and expensive. Moreover, various overtones occur, which are often indicated by the device instead of the fundamental resonant tone indicating the web tension. The result on a measuring device operating according to the above-mentioned principles will then be quite misleading. Therefore, these methods have not been used to a large extent in practice.